The present invention relates to two-dimensional image detectors capable of detecting images by means of X-rays and other kind of radiation and visible, infrared, and other light.
Conventionally known two-dimensional image detectors for detecting images of radiation, such as X-rays, or light, such as visible or infrared light, form an image by collecting and reading electric charges generated in a photosensitive semiconductor layer by means of pixels located on an active matrix substrate such as the one used in a conventional type of liquid crystal display device. For the purpose of increasing the per-pixel fill factor (aperture ratio), some conventional two-dimensional detectors have a so-called roof structure in which pixel electrodes are separated by an insulating layer from the address lines (electrode wires) and TFT elements that are provided in turn on the active matrix substrate.
More specific description of the structure can be found in Similarities between TFT Arrays for Direct-Conversion X-ray Sensors and High-Aperture AMLCDs, by W. den Boer, et al., SID 98 DIGEST, PP.371-374, 1998; and U.S. Pat. No. 5,780,871 (Date of patent: Jul. 14, 1998).
FIG. 9 is a cross-sectional view schematically showing an arrangement of a pixel in a two-dimensional image detector having a conventional roof-structured active matrix substrate.
As shown in FIG. 9, in the base structure of the two-dimensional image detector, there are provided a roof-structured active matrix substrate 101, a photoconductor film 102, and a common electrode 103, forming sequential layers in this order.
The roof-structured active matrix substrate 101 includes a glass substrate 104, a TFT (Thin Film Transistors) 105, an electric charge storage capacitance (Cs) 106, and a pixel electrode 107. The TFT 105 acts as a switching element. The pixel electrode 107 caps an insulating layer 115 which in turn covers the TFT 105 and the electric charge storage capacitance (Cs) 106.
The TFT 105 is constituted by a gate electrode 108, a gate insulating film 109, an a-Si film (i layer) 110, a-Si film (n+ layer) 111, a source electrode 112, and a drain electrode 113. The electric charge storage capacitance (Cs) 106 is constituted by a storage capacitance electrode (Cs electrode) 114, a gate insulating film 109, and a drain electrode 113 that also acts as a storage capacitance electrode forming a pair with the storage capacitance electrode (Cs electrode) 114.
The insulating layer 115 is interposed so as to electrically insulate the pixel electrode 107 from the electrode wires (the gate electrode 108 and the source electrode 112), the TFT 105, the electric charge storage capacitance (Cs) 106. The pixel electrode 107 and the drain electrode 113 are electrically connected in a contact hole 116 formed through the insulating layer 115.
The photoconductor film 102 is composed of a semiconductor material that generates electric charges (electrons, holes) when irradiated with radiation such as X-rays or light such as visible light.
Now, principles in the operation of the two-dimensional image detector will be discussed.
Electric charges (electrons, holes) are generated in the photoconductor film 102 when radiation such as X-rays or light such as visible light is projected to the photoconductor film 102 with voltage being applied across the common electrode 103 and the storage capacitance electrode (Cs electrode) 114. The generated electric charges move toward either the positive or negative electrode according to the sense of the apply voltage and are stored in the electric charge storage capacitance (Cs) 106. The electric charges stored in the electric charge storage capacitance (Cs) 106 are available for output through the source electrode 112 if the TFT 105 is changed to an open state by an input signal to the gate electrode 108.
The electrode wires (gate electrode 108 and source electrode 112), the TFT 105, and the electric charge storage capacitance (Cs) 106, when provided to form a X-Y matrix, are capable of scanning input signals to the gate electrodes 108 sequentially row by row and thereby obtain image information in two dimensions.
In two-dimensional image detectors including the aforementioned roof-structured active matrix substrate, an insulating layer is interposed between the pixel electrodes and the address lines (electrode wires) to provide insulation between them. FIG. 9 shows the insulating film 115 as an example of such an insulating layer. The insulating layer may be composed of a material such as, SiOx, SiNx, Al2O3, polyimide, or an acrylic resin; however, the level of insulation these materials offer varies from material to material for the following reasons.
Resin films can be formed using a spin coating, film lamination, or other similar technique. SiOx and SiNx. films are costly, in comparison with resin films, due to the use of CVD (Chemical Vapor Deposition) in the manufacturing process. Resin films can be readily fabricated so as to make a flat surface on them using a spin coating technique, etc. By contrast, SiOx and SiNx films deposited using a CVD technique are inevitably affected by irregularities in the underlying layer, and will have a surface which is far from being flat in a satisfactory manner. In two-dimensional image detectors, irregularities in the surface of an insulating layer adversely affect the photoconductive layer deposited on the active matrix substrate, which undesirably degrades its detection performance. Thus, resins are the preferred material to form an insulating layer with a flat surface.
In two-dimensional image detectors, a parasitic capacitance appears where the address line (electrode wire) is placed on the top of on a pixel electrode, which is a major factor causing noise in signals. To reduce the signal noise, the parasitic capacitances should be reduced. A preferred insulating layer is therefore a thick one as long as other conditions allow. CVD techniques are hardly capable of forming a film thicker than 1 xcexcm; by contrast, thick resin films are readily formed by spin coating. Further, a typical resin have a low dielectric constant, and allows reduction of the parasitic capacitances.
Further, contact holes should be provided in the insulating layer, in which the pixel electrodes are connected to the drain electrodes. The contact holes are formed using a photolithography technique. In view of these facts, the insulating film composed of an acrylic resin or other photosensitive material is convenient, because such an insulating film can be subjected to a photolithography process to form the contact holes, without applying and etching resist, and contributes to speedy processing in comparison with non-photosensitive materials.
For these reasons, the insulating layer is preferably made of an acrylic or other similar resin with a low dielectric constant and a satisfactory level of photosensitivity, and fabricated using a spin coating technique. To reduce the parasitic capacitances, the insulating layer is preferably made of a material with a low dielectric constant.
However, the following problems entail if a conventional two-dimensional image detector has a roof-structured active matrix substrate with an insulating layer made of the acrylic or other similar resin.
If the resin insulating layer included in a conventional roof-structured active matrix substrate is in direct contact with ambient air along the edges of a pixel area, degradation of the insulating layer material possibly greatly affects the reliability of the device. Specifically, we are concerned about ambient humidity among other factors, which is a likely cause to adversely affect the reliability. In general, acrylic and other similar resins are not durable in the presence of humidity; therefore, if composed of such a resin, the insulating layer peels off and inevitably degrades or otherwise changes in properties with time, starting where the insulating layer is in direct contact with ambient humidity. Acrylic and other similar resins have further problems that they are easy to polymerize, decompose or otherwise degrade due to irradiation by, for example, X-rays.
The present invention has an object to offer a highly reliable two-dimensional image detector, including a photoconductive semiconductor layer deposited on a roof-structured active matrix substrate, of which the insulating layer is protected from decay where it is in direct contact with ambient air along the edges of a pixel area so as to maintain its performance as a device.
In order to achieve the object, a two-dimensional image detector in accordance with the present invention includes:
an active matrix substrate including pixel electrodes provided on an insulating layer provided in turn on;
electrode wires arranged in a matrix;
switching elements located at respective points in the matrix; and
electric charge storage capacitances connected respectively to the electrode wires via the switching elements;
a photoconductive semiconductor layer disposed on the active matrix substrate; and
a protection film provided so as to cover edges of the insulating layer.
According to the arrangement, in the active matrix substrate, there is provided a protection film covering edges of the insulating layer interposed so as to electrically isolate the pixel electrodes from the electrode wires, the switching elements, and the electric charge storage capacitances; therefore, the edges of the insulating layer are not exposed, and the insulating layer is not exposed to external atmosphere. Consequently, even if the insulating layer is composed of, for example, an acrylic or other resin which is generally not durable in the presence of humidity, the insulating layer is prevented from peeling off, degrading, or otherwise changing in properties with time due to humidity and other adverse effects of ambient air starting where the insulating layer is exposed to ambient air, and also prevented from polymerizing, decomposing or otherwise degrading due to direct irradiation by, for example, X-rays.
This enables a satisfactory level of prevention of the deterioration of the material composing the insulating layer due to exposure of its exposed parts to external atmosphere and due to irradiation by, for example, X-rays during detection operation.
Alternatively, the semiconductor layer may double as the protection film.
A typical two-dimensional image detector includes a photoconductive semiconductor layer provided on an active matrix substrate. Electric charges (electrons, holes) are generated in a photoconductive semiconductor layer as it is irradiated by radiation or visual light. The electric charges (electrons, holes) are then collected in pixels on the active matrix substrate, and an image is formed from the readings. In the foregoing arrangement, a photoconductive semiconductor layer of the type used in a conventional two-dimensional image detector is provided so as to cover the edges of the insulating layer and doubles as a protection film to the insulating layer, which eliminates the need to provide a separate protection film for protecting the edges of the insulating layer.
This, without increasing the number of steps in manufacture, enables a satisfactory level of prevention of the deterioration of the material composing the insulating layer due to exposure of its exposed parts to external atmosphere and due to irradiation by, for example, X-rays during detection operation.
Preferably, on the semiconductor layer, there is additionally provided an electrode layer stretching only within the confines of the insulating layer.
Although the efficiency for the semiconductor layer to covert electric charges varies depending on the material, the semiconductor layer generally needs to be relatively thick to ensure a certain level of electric charge conversion efficiency. For example, an a-Se semiconductor layer needs to be 0.5 mm to 1.5 mm thick, in which case the electrode layer receives a voltage as high as a few kilovolts to tens of kilovolts.
In the foregoing arrangement in accordance with the present invention, no electrode layer is provided in an area where the insulating layer does not exist; therefore, the semiconductor layer, being provided in an area where the insulating layer does not exist, does not receive a high voltage from the electrode layer. Consequently, the insulating film typically provided on the electrode wires where the insulating layer does not exist does not need to provide isolation from any high voltage. This enables the prevention of isolation breakdown on the electrode wires and generation of parasitic capacitances in the insulating film disposed on the electrode wires. Thus, signal noise is reduced, and the reliability of the device is improved.
Further, the protection film may be composed of a resin.
In the arrangement, the protection film for protecting the edges of the insulating layer is composed of a resin; therefore a thick protection film is readily formed. In addition, the protection film is preferably insulating. Since a typical resin has a low dielectric constant, a protection film, if composed of such a resin, has a reduced dielectric constant.
This readily ensures a satisfactory level of prevention of the deterioration of the material composing the insulating layer due to exposure of its exposed parts to external atmosphere and due to irradiation by, for example, X-rays during detection operation.
Alternatively, in order to achieve the object, another two-dimensional image detector in accordance with the present invention includes:
an active matrix substrate including pixel electrodes provided on an insulating layer provided in turn on:
electrode wires arranged in a matrix;
switching elements located at respective points in the matrix; and
electric charge storage capacitances connected respectively to the electrode wires via the switching elements; and
a shield member for shielding the insulating layer from external atmosphere.
According to the arrangement, in the active matrix substrate, there is provided a shield member for shielding, from external atmosphere, the insulating layer interposed so as to electrically isolate the pixel electrodes from the electrode wires, the switching elements, and the electric charge storage capacitances; therefore, the insulating layer is not exposed to external atmosphere. Consequently, even if the insulating layer is composed of, for example, an acrylic or other resin which is generally not durable in the presence of humidity, the insulating layer is prevented from peeling off, degrading, or otherwise changing in properties with time due to humidity and other adverse effects of ambient air, starting where the insulating layer is exposed to ambient air, and also prevented from polymerizing, decomposing or otherwise degrading due to direct irradiation by, for example, X-rays.
This enables a satisfactory level of prevention of the deterioration of the material composing the insulating layer due to exposure of its exposed parts to external atmosphere and due to irradiation by, for example, X-rays during detection operation.
Preferably, the shield member maintains the insulating layer in either one of inert gas, dry gas, and depressurized atmospheres so as to provide shield to the insulating layer from external atmosphere.
According to the arrangement, the insulating layer is maintained in either one of inert gas, dry gas, and depressurized atmospheres. It is therefore ensured that the insulating layer, especially the exposed parts, is shielded from humid external atmosphere.
This enables a satisfactory level of prevention of peeling of the insulating layer and deterioration of the material due to humidity.
Preferably, in each arrangement above, the insulating layer is composed of a resin.
According to the arrangement, the insulating layer, being composed of a resin, can be formed by a spin coating, film lamination, or other similar technique; therefore, the insulating layer can be readily formed with a flat surface and an increased thickness. Since a typical resin has a low dielectric constant, the insulating layer, if composed of such a resin, has a reduced dielectric constant.
Thus, a satisfactory level of prevention of poor detection performance, reduction in parasitic capacitances, and less likelihood of signal noise generation are imparted to two-dimensional image detectors at low costs.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, are not in any way intended to limit the scope of the claims of the present invention.